Both plants and animals use steroids as signaling molecules to regulate growth and development. While animals often use nuclear receptors for the perception and signal transduction of their steroid hormones, plants rely on cell surface receptors for steroid detection and a phosphorylation cascade to relay the brassinosteroid (BR) signals into the nucleus. Over the last several years, several key regulators of BR signaling have been identified, including BRI1 and BAK1, 2 interacting receptor kinases that function together to mediate steroid perception at the cell surface, BIN2, a GSK3-like kinase that negatively regulates the intracellular transduction of the BR signals, BSU1, a nuclear-localized protein phosphatase that antagonizes the action of BIN2, BES1 and BZR1, 2 BIN2 substrates that control the expression of many BR-responsive genes in the nucleus. Despite these findings, our knowledge on the signaling mechanism of the plant steroid hormones is rather limited. The long-term goal of our research is to delineate the entire plant steroid signaling pathway, and the current proposal is designed to fill 2 main gaps in our current knowledge of the BR signaling mechanism and identify additional regulators of the pathway. A combinatory approach of biochemistry, genetics, and molecular biology, including protein purification, yeast 2-hybrid method, chemical genetics, and genetic interactive screens, will be used 1) To determine how the BRI1-containing BR receptor transmits the plant steroid signals into plant cells, 2) To clone novel BRI1 suppressor genes and investigate the physiological functions of their genes products, 3) To study the biochemical mechanism by which the BIN2 GSK3 kinase is regulated by BRs, and 4) To identify key regulators of the BR-activated BIN2 regulatory mechanism. The successful execution of this proposal will significantly increase our understanding of the plant steroid signaling mechanism, eventually leading to new strategies to improve crop quality and productivity. Given the ubiquity of the GSK3 kinase and the existence of membrane-initiated animal steroid signaling, the discoveries resulted from this proposal may lead to the elucidation of new features of GSK3 regulation and identification of new regulators of steroid action in humans.